Many types of electronic circuits, such as digital-to-analog converters, voltage regulators and precision amplifiers, require a temperature-independent bias reference. The stable reference can be either a current or a voltage. For most applications, a voltage reference is preferred because it easily can be interfaced with other circuits.
High precision voltage references can achieve temperature stability of less than 10 parts-per-million per degree centigrade (ppm/.degree. C.). Temperature compensation in monolithic integrated circuit design typically involves compensating a voltage source of predictable negative temperature drift with another voltage source of predictable positive temperature drift.
For example, the base-emitter drop (V.sub.BE) of a bipolar transistor has a negative temperature coefficient of about -0.002 volts/.degree. C. (V/.degree. C.). By contrast, the temperature dependence of the V.sub.BE difference (.increment.V.sub.BE) is proportional to absolute temperature through the thermal voltage V.sub.T. A "band-gap" reference operates by generating a first temperature drift due to V.sub.BE, generating a second temperature drift proportional to V.sub.T and then adding the first and second temperature drifts to obtain a nominally zero temperature dependence.
Commercially manufactured integrated circuit voltage references must provide a specified output voltage at a specified temperature coefficient. The temperature coefficient, however, is difficult to measure quickly, economically and accurately. A highly accurate way to measure the temperature coefficient of a voltage reference is to place the circuit in an oven, and then monitor the circuit's output voltage while changing temperature. This technique is time consuming, however, because the temperature of the oven interior and the circuit must stabilize for each temperature at which a measurement is required.
Measurement time can be reduced by heating only the die or the package. For example, in one previously known technique, power transistors in the voltage reference are turned ON to dissipate power and thereby heat the die. Because the power transistors generally are located in a limited portion of the die, however, this technique creates localized heating and large temperature gradients across the die. As a result, accurate measurements cannot be taken until the temperature gradient dissipates.
In another previously known technique, implemented on the LT1019 Precision Reference manufactured by Linear Technology Corporation, Milpitas, Calif., a heater resistor coupled between a "HEATER" pin and GROUND is fabricated in the silicon substrate along with the voltage reference circuits. The heater resistor has a nominal value of 400 ohms (.OMEGA.) and is implemented as a diffusion resistor. Because diffusion resistors have relatively high resistivity per unit area (typically specified in ohms-per-square (.OMEGA./square)), the heater resistor on the LT1019 consumes a large area and therefore only is implemented along one side of the circuit die. As a result, the diffusion resistor heater also creates localized heating and temperature gradients across the die.
As previously mentioned, accurate temperature coefficient measurements cannot be taken until die temperature gradients dissipate. For packaged integrated circuits, the gradient dissipation time depends on, among other things, the thermal characteristics of the package. Large packages that contain large amounts of high thermal resistance material surrounding the die, such as dual in-line packages (DIP), have long thermal time constants. Because the time constant of the die temperature gradients is much shorter than the thermal time constant of such large packages, measurements can be taken before the die temperature drops significantly. Smaller packages, however, such as the miniature small-outline package (MSOP) and SOT-23 surface mount package, have shorter thermal time constants, and the die temperature drops too quickly before the die temperature gradients settle.
It therefore would be desirable to provide methods and apparatus for quickly and efficiently heating an integrated circuit die.
It also would be desirable to provide methods and apparatus for uniformly heating an integrated circuit die with minimal thermal gradients across the die.
It further would be desirable to provide methods and apparatus for quickly and uniformly heating an integrated circuit die in a packaged part.